1. Technical Field
Example embodiments of the present invention relate in general to an apparatus and method for correcting distortion of an image, and more specifically, to a method of correcting distortion that has occurred in an image and an apparatus using the method.
2. Related Art
With the development of high-capacity memory chips and the drastic increase in computer processing speed, computer-based image processing technology is being widely used in general industries. In particular, the technology is becoming essential component technology for examination and measurement in factory automation lines, web-based real-time monitoring systems, etc. When a wide-angle lens such as a fisheye lens is used, an image of a large area can be obtained in the same field of view (FOV). However, as the distance from the center of an image increases, distortion becomes severe, and thus geometric correction is needed.
In connection with correction of an input image, many researchers have reported distorted image correction technology for cameras, video cassette recorders (VCRs), etc. To correct distortion caused by a wide-angle lens, Rahul has researched a camera correction method for correcting both of radial distortion and decentering distortion. Weng has researched an algorithm for correcting an image input from a camera. Tsai has reported a first fast Fourier transform (FFT)-based correction formula, and corrected an image center point using a rigid body transformation, a distance equation for a pinhole camera, and a radial distortion formula expressed by a second-order term.
Most researchers have used a general formula of a correction model for distortion occurring in a diagonal direction, etc. of a camera whose wide angle is not large, etc., and performed correction by calculating respective coefficients using a minimizing method. Also, due to complexity of a general formula for a correction model, most researchers have ignored high-order terms and applied the correction model only to a specific distortion type in a specific vision environment.
Aberration is an important factor that determines performance of an optical system. Aberration is caused by a characteristic of a lens of a uniform curvature and refractive index, denoting a phenomenon in which an image is not normally formed and thus looks blurred or distorted.
In general, lens aberrations are classified into five types: spherical aberration, coma, astigmatism, curvature of field, and distortion, which are referred to as the Seidel aberrations.
Spherical aberration denotes a phenomenon in which a ray incident from a position far from a vertical axis passing through the center of a lens, that is, an on-axis, is refracted more than a ray incident from a position close to the on-axis, and forms an image at a position close to the lens.
Astigmatism is caused by poor grinding of a lens, a faulty lens material, temperature difference, etc.
Curvature of field denotes a phenomenon in which a ray incident from a position far from an on-axis of a lens forms an image at a position close to the lens, and thus the focus is not on a plane but on a spherical surface, resulting in a curved image.
Distortion denotes a phenomenon in which a straight line of a subject forms a curved image.
Color aberration denotes a phenomenon in which a subject appears to spread according to colors. When light passes through a lens, rays of the light are refracted according to their wavelengths. Thus, rays having relatively short wavelengths are more refracted than others, that is, rays having different wavelengths differently form an image.
Radial distortion frequently occurs in a camera equipped with a cheap lens, particularly, a fisheye lens or tilt lens. Such lens distortion occurs in all image apparatuses using a lens. When a center point of such lens distortion is not found, it is impossible to correct the lens distortion, or new distortion occurs even after the distortion is corrected. The problem of image distortion is very important for digital imaging system manufacturers and core component providers.
To reduce lens distortion, a special material may be used for a lens, or several lenses may be attached to each other. However, the former method causes an increase in the price of a lens, and thus cannot be applied to a small camera whose lens cannot be changed. Also, when several lenses are used, a distortion center is warped.
Due to these problems, many methods for correcting such distortion by image processing are under development. Thus far, distortion correction technology has mainly used a camera setting for photographing or an image of a specific test pattern. Such distortion correction technology is being gradually succeeded by research on an automatic correction method of correcting lens distortion using only image information without additional information.
Most conventional methods concentrate on a technique for correcting radial lens distortion that is easy to analyze and correct. In particular, research on lens distortion correction techniques based on image processing techniques has been carried out to correct lens distortion of a small camera. Mainly, research has been conducted on techniques for correcting radial lens distortion by finding the degree of distortion on the basis of a center point of the distortion. Optimization is performed to solve a set of all equations describing a set of curves and aberration between a set of straight lines and the set of curves, thereby calculating the overall degree of distortion from a distorted image to a corrected image.